| ||For the modified FEV strain in Fallout 2, see FEV Curling-13.|
The modified FEV is a quest item in Fallout 3. It is given to the Lone Wanderer by John Henry Eden in Raven Rock and can be inserted in the Auxiliary Filtration Input in Jefferson Memorial's Rotunda, contaminating the purifier's filter system and subsequently, the Capital Wasteland's waters during Take it Back!.
The modified FEV is a heavily modified sub-strain of the Forced Evolutionary Virus that was developed by John Henry Eden. It is said to be based on or related to a progenitor, the FEV Curling-13 sub-strain the Enclave's Chemical Corps had originally synthesized and tested in 2242.
Some changes include its intended design of being compatibly inserted into a water purification system's schematics, such as the one found at the Jefferson Memorial. Also, highly unlike the FEV Curling-13, it does not target those that are pure humans.
According to President Eden, it will indiscriminately target and exterminate super mutants, ghouls, any and all other mutated creatures, and generally anything that has been at all affected by Wasteland mutation upon ingestion. This would include any wastelander that had been originally born in the Wastelands and/or heavily exposed to its background radiation and other mutational hazards, or hosts any other sort of mutational side effect at all as a result.
Theoretically, the only people who would prove immune to infection are dangerous radiation level/mutation free pure humans, those that are born inside Vault-Tec vaults or those born inside other similarly sealed environments and as a result, unaffected by Wasteland mutation. This is proven to be accurate in one of the ending slides if the player completes Project Impurity. 
In the add-on Broken Steel, if the Lone Wanderer chose to contaminate the Purifier with the modified FEV, the medical facility of nearly every major settlement in the Capital Wasteland (except Tenpenny Tower and Paradise Falls) has at least two or three newly arrived sick patients lying on the beds, dying from modified FEV infection.
The patients hold their stomachs and moan that "it burns on the inside." Doctor Preston from Rivet City states that he is fairly certain that 'the condition' is not contagious to his knowledge. However, he does state that Rivet City is burning the corpses of the infected dead just to be safe, out of fearful precaution that drinking the water might indeed transmit or otherwise result in a contagious plague of some kind developing as a result.
Furthermore, Doctor Preston states that the Modified FEV causes the patient's immune system to begin attacking the body, killing them in the process.
- With Broken Steel and if the Lone Wanderer drinks three successive bottles of Aqua Pura after having completed Project Impurity and inserting the modified FEV, it kills them.
- It is not possible to leave the room where President Eden is located unless the Wanderer has picked up and retrieved the modified FEV from Eden's console.
- This item uses the world model of an empty Nuka-Cola bottle as a placeholder, likely because, as a quest item, it can never be dropped normally.
- It can be given to Elder Lyons for good Karma if he is spoken to after the escape from Raven Rock. With Broken Steel, it will be removed from inventory if Project Purity is activated without the FEV being given to Elder Lyons beforehand.